He ascended the throne after a long internecine war against Sweyn III and Canute V. The Slav nomadic tribes of the Wends took advantage of this war and made several devastating incursions to Jutland and Danish islands. When Valdemar I took the throne, he concentrated all his forces on the struggle against the nomads. He gained support of the Duke Henry the Lion of Saxony and Bavaria. To strengthen his relations with him, Valdemar signed a treaty (1164) according to which his son Canute IV was to marry Henry’s daughter Rikissa. But in 1167 she died and Canute married another daughter of Henry – Gertrude. Valdemar also established close diplomatic relations with Friedrich Barbarossa, Emperor of the Holy Roman Empire.
In 1154 he married Sophia Volodymyrivna, daughter of the Prince Volodymyr Vsevolodovych of Novgorod (or the Prince Volodar Hlibovych of Minsk) and Rikissa, daughter of Duke Boleslaw III. They had the following children: King Canute VI of Denmark, King Valdemar II of Denmark, Ingeborg (wife of King Philip II of France), Rikissa (wife of King Eric X of Sweden), and others.
During his reign, Valdemar carried on about twenty land and sea campaigns. He was supported by the church, as besides territorial expansion he contributed to the dissemination of Christianity. His most successful campaign was in 1168, when Arkona and the Island of Rügen in the Baltic Sea were taken. Valdemar waged several campaigns to Norway, where he wanted to enthrone his son Magnus. In 1171, Valdemar crowned his